Heretofore, various devices have been utilized to control the rate of relative movement between first and second members. Such devices, however, are particularly useful in loading dock equipment such as dock levelers. A conventional dock leveler compensates for the height differential which might occur between the dock platform and the bed of a vehicle while parked at the loading dock for loading or unloading thereof. The dock leveler is mounted within a pit formed in the dock platform and has an open top and an open front side, the latter being disposed at the front wall of the dock adjacent the parked vehicle. The leveler includes a heavy dockboard which overlies the pit open top and is hingedly connected to the dock adjacent the rear wall of the pit. Hingedly connected to the front edge of the dockboard, which is adjacent the pit open front side, is an extension plate. The plate is adapted to move relative to the dockboard front edge between a depending inoperative position and an extended operative position. When the extension plate is in its operative position, it is extending outwardly from the dockboard front edge and may be substantially coplanar with the upper surface of the dockboard and rest upon and be supported by the upper surface of the bed of the parked vehicle. Thus the extended extension plate spans the gap which is formed between the rear edge of the vehicle bed and the front edge of the dockboard.
To move the extension plate from a depending inoperative position to an extended position normally requires the dockboard to be pivoted upwardly a sufficient amount whereby the extension plate can swing outwardly by some independent force to the extended position without obstruction from the parked vehicle. Once the extension plate has cleared the parked vehicle, the dockboard and extended extension plate are pivoted downwardly as a unit until the extended extension plate rests upon the vehicle bed. After loading or unloading of the parked vehicle has been completed, the vehicle is moved away from the dock causing the extension plate to assume its depending position. It is important that the extension plate assumes its proper depending position whereby the dockboard is supported thereby in a horizontal coplanar relation, or cross-traffic position, with the dock platform. When the dockboard is being supported by the depending extension plate the latter engages stationary lugs or cleats protruding from the dock front wall.
Because the extension plate normally relies on gravitational force to move from the extended position to the fully dependent position, difficulty has heretofore been frequently encountered in having sufficient gravitational force to move the extension plate the necessary amount after the plate has pivoted through a predetermined sector (i.e. the first half of its downward travel). To prevent premature pivotal movement of the extension plate from its extended position before it is brought into supporting engagement with the bed of the parked vehicle, a hydraulic device similar to a shock absorber was utilized for this purpose.
Such prior devices utilized for this purpose were beset with one or more of the following shortcomings: (a) to achieve sufficient falling time of the plate during the first half to its downward pivoting motion of the device could not prevent the extension plate from becoming hung up in a partially extended position due to loss of gravitational force of the plate thereby preventing the dockboard from being properly supported in a cross-traffic position by the extension plate; (b) the device was highly susceptible to malfunction; (c) the device was difficult and awkward to install and maintain in proper working order; and (d) the device would not function properly under extreme climatic conditions.